It is well known to use diamonds to form hard, abrasion resistant and erosion resistant coatings or surfaces on cutting tools, bearings, drill bits, nozzles, valve seats and the like. There are several types of surfacing and supporting assemblies which utilize diamond as a constituent. In one type, the diamonds are a very small size and randomly distributed in a supporting matrix. Another type includes diamonds of a larger size positioned on the surface of a supporting member in a predetermined pattern. Still another type involves the use of a surface formed of a polycrystalline diamond supported on a sintered carbide or other type of support member (PDC). This support member may be, as an example, a cutting tool structure, or a drill bit structure.
Diamonds are an allotropic form of carbon, which is crystallized isometrically. It consists of carbon atoms covalently bound by single bonds only in a predominantly octahedral structure. This accounts for its extreme hardness (Mohs 10) and great stability. It has a specific gravity of 1.5 and a coefficient of friction of 0.05. Diamonds will melt at 3700 degrees centigrade (.degree.C). They can also be made synthetically by heating carbon and a metal catalyst in an electric furnace at about 3000.degree. F. under pressure of about 1.0 million pounds per square inch (psi).
Carbide is a binary solid compound of carbon and another element. The most familiar carbides are those of calcium, tungsten, boron, and iron (cementite). Two factors have an important bearing on the properties of carbides: (1) the difference in electronegativity between carbon and the second element, and (2) whether or not the second element is a transition metal. A "cemented carbide" is formed from a powdered form of refractory carbide which is united by compression with a bonding material (usually iron, nickel, or cobalt), followed by sintering. For example, tungsten carbide is bonded with 3 to 25 percent cobalt at 1400.degree. C. Cemented carbide is used chiefly in metal cutting tools, which are hard enough to permit cutting speeds in rock or metal up to 100 times that obtained with alloy steel tools.
Boron nitride (BN) occurs as a white powder, with a particle size of about 1 micron, having a graphite-like hexagonal plate structure which melts at 3000.degree. C. When compressed at a million psi, it becomes as half as hard as diamond. The resulting material has excellent heat-shock resistance.
It should be understood that the term polycrystalline diamond (PCD) or polycrystalline diamond compact (PDC) or sintered diamond, as the material is often referred to in the literature, can also be any of the super hard materials, including, but not limited to synthetic or natural diamond, cubic boron nitride (CBN), and wurtzite boron nitride as well as combinations thereof. Also, cemented metal carbide refers to a carbide of one of the group IVB, VB, or VIB metals which is pressed and sintered in the presence of a binder of cobalt, nickel, or iron and the alloys thereof.
As discussed in U.S. Pat. No. 4,255,165, a cluster compact is defined as a cluster of abrasive particles bonded together either (1) in a self-bonded relationship, (2) by means of a bonding medium disposed between the crystals, or (3) by means of some combination of (1) and (2). Reference can be made to U.S. Pat. Nos. 3,136,615; 3,233,988 and 3,609,818 for a detailed disclosure of certain types of compacts and methods for making such compacts. All of these teachings specify the use of high temperature combined with high pressure for a relatively long period of time to form the various compacts.
A composite compact is defined as a cluster compact bonded to a substrate material such as cemented tungsten carbide. A bond to the substrate can be formed either during or subsequent to the formation of the cluster compact. It is, however, again highly preferable to form the bond at high temperatures and high pressures, and for a time period comparable to those at which the cluster compact is formed. Reference can be made to U.S. Pat. No. 3,745,623 for a detailed disclosure of certain types of composite compacts and methods for making same. The composite compact is then attached to a support structure such as the metallic body or shank of a cutting tool.
As discussed in U.S. Pat. No. 5,011,515, composite polycrystalline diamond compacts, PDC, have been used for industrial applications including rock drilling and metal machining for many years. As an example, the composite compact consisting of PDC and sintered substrate are affixed as insert elements in a rock drill bit structure. One of the factors limiting the success of PCD is the strength of the bond between the polycrystalline diamond layer and a sintered metal carbide substrate. It is taught that both the PDC and the supporting sintered metal support substrate must be exposed to high pressure and high temperature, for a relatively long period of time, in order to achieve the desired hardness of the PDC surface and the desired strength in the bond between the PDC and the support substrate.
U.S. Pat. No. 3,745,623 (reissue U.S. Pat. No. 32,380) teaches the attachment of diamond to tungsten carbide support material with an abrupt transition there between. This, however, results in a cutting tool with a relatively low impact resistance. Due to the differences in the thermal expansion of diamond in the PCD layer and the binder metal used to cement the metal carbide substrate, there exists a shear stress in excess of 200,000 psi between these two layers. The force exerted by this stress must be overcome by the extremely thin layer of cobalt which is the common or preferred binding medium that holds the PDC layer to the metal carbide substrate. Because of the very high stress between the two layers which have a flat and relatively narrow transition zone, it is relatively easy for the compact to delaminate in this area upon impact. Additionally, it has been known that delamination can also occur on heating or other disturbances in addition to impact. In fact, parts have delaminated without any known provocation, most probably as a result of a defect within the interface or body of the PDC which initiates a crack and results in catastrophic failure.
One solution to the PDC-substrate binding problem is proposed in the teaching of U.S. Pat. No. 4,604,106. This patent utilizes one or more transitional layers incorporating powdered mixtures with various percentages of diamond, tungsten carbide, and cobalt to distribute the stress caused by the difference in thermal expansion over a larger area. A problem with this solution is that "sweep-through" of the metallic catalyst sintering agent is impeded by the free cobalt and the cobalt cemented carbide in the mixture. In addition, as in previous referenced methods and apparatus, high temperatures and high pressures are required for a relatively long time period in order to obtain the assembly disclosed in U.S. Pat. No. 4,604,106. Pressures and temperatures are such that, using mixtures specified, the adjacent diamond crystals are bonded together.
U.S. Pat. No. 4,784,023 teaches the grooving of polycrystalline diamond substrates but it does not teach the use of patterned substrates designed to uniformly reduce the stress between the polycrystalline diamond layer and the substrate support layer. In fact, this patent specifically mentions the use of undercut (or dovetail) portions of substrate ridges, which solution actually contributes to increased localized stress. Instead of reducing the stress between the polycrystalline diamond layer and the metallic substrate, this actually makes the situation much worse. This is because the larger volume of metal at the top of the ridge will expand and contract during temperature cycles to a greater extent than the polycrystalline diamond, causing the composite to fracture at the interface. As a result, construction of a polycrystalline diamond cutter following the teachings provided by U.S. Pat. No. 4,784,023 is not suitable for cutting applications where repeated high impact forces are encountered, such as in percussive drilling, nor in applications where extreme thermal shock is a consideration.
By design, all of the cutting surfaces disclosed in the above references are "hard" in that they are abrasion and erosion resistant. This is particularly true for PDC material which is also quite brittle and subject to fracturing upon impact. Because of the brittleness and overall hardness, it is not practical and economical to mold or machine surfaces of tools, bearings and the like made of PDC in the manufacturing process for these devices. Alternately, the PDC surfaces are preferably "molded" or preformed using techniques taught in U.S. Pat. No. 4,662,896.
In summary, prior art teaches the manufacture and the use of various abrasion and erosion resistant materials to form inserts which are used as wear surfaces for machine tools, drill bits, bearings, and other similar surfaces. All of the processes in the cited references require high temperatures and high pressures for a relatively long period of time to form the wear resistant surface material, or to bond the wear resistant surface material to the underlying support substrate, or both. Furthermore, the bond between surface and substrate of the resulting inserts is subject to weakening due to differences in thermal expansion properties which become a factor as the device heats up during use. These surfaces are formed under high pressure, temperature, and application time to form a surface which is quite hard and durable, but which is also quite brittle, subject to fracturing upon impact, and are in general very difficult to handle in the manufacturing process of tools employing such wear resistant surfaces. High temperature and high pressure equipment used in the manufacture of devices employing PDC are quite expensive to obtain and to maintain.